Glass-ceramic composition for recording disk substrate

ABSTRACT

A glass ceramics composition for recording disk substrate consists essentially, expressed in terms of weight percent on the oxide basis, of from 65 to 80 wt % of SiO 2 , from 3 to 15 wt % of Al 2 O 3 , from 3 to 15 wt % of Li 2 O, from 0.2 to 5 wt % of P 2 O 5 , and from 0.1 to 0.8 wt % of ZrO 2 .

RELATED APPLICATION

This application is based on application No. 11-191743 filed in Japan,the content of which is hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a glass ceramic composition, moreparticularly, relates to the glass ceramic composition suitable formagnetic disk substrate.

DESCRIPTION OF THE PRIOR ART

Magnetic disks are mainly used as recording media of computers. Aluminumalloys have heretofore been used as the material of magnetic disksubstrates. However, in the recent trend for a smaller size, a thinnerthickness, and a higher recording density of magnetic disks, a highersurface flatness and a higher surface smoothness are increasinglydesired. Aluminum alloys cannot satisfy the desire, and a material formagnetic disk substrates which can replace aluminum alloys is required.Thus, in particular, recent attention has been focused on the glasssubstrate for the disk because of its surface flatness and smoothnessand excellent mechanical strength.

As glass substrates for disks for recording media, there have beenproposed a chemically reinforced glass substrate having a surfacereinforced by ion exchange or like method and a glass ceramics substrateon which a crystal component has been precipitated to reinforce thebonding. In recent years, the latter crystallized glass substrate inwhich a crystallite has been precipitated in glass by heat treatment hasdrawn particular attention because of its excellent strength and highproductivity.

As recent requirements on the performance of a disk for a recordingmedium have been more stringent, a substrate material has also beenrequired to have an increased strength related directly to the bendingor warping of the disk during high-speed rotation. The strength can berepresented by the elastic modulus ratio (=Young's modulus/specificgravity) of the substrate material. The elastic modulus ratio having ahigher value indicates a higher mechanical strength. However, aglass-ceramics composition conventionally known has the problem that theproductivity thereof is reduced significantly if the strength thereof isto be increased.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a glass ceramiccomposition which is suitable for use in an improved glass substrate fora recording medium.

Another object of the present invention is to provide a glass ceramiccomposition which has high productivity irrespective of its high elasticmodulus ratio.

Still another object of the present invention is to provide a disksubstrate for a recording medium which has high productivityirrespective of its high elastic modulus ratio.

Thus, the present invention provides a glass ceramics composition forrecording disk substrate consisting essentially, expressed in terms ofweight percent on the oxide basis, of from 65 to 80 wt % of SiO₂, from 3to 15 wt % of Al₂O₃, from 3 to 15 wt % of Li₂O, from 0.2 to 5 wt % ofP₂O₅, and from 0.1 to 0.8 wt % of ZrO₂.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides a glass ceramics composition forrecording disk substrate consisting essentially, expressed in terms ofweight percent on the oxide basis, of from 65 to 80 wt % of SiO₂, from 3to 15 wt % of Al₂O₃, from 3 to 15 wt % of Li₂O, from 0.2 to 5 wt % ofP₂O₅, and from 0.1 to 0.8 wt % of ZrO₂.

In the composition, SiO₂ is a glass network former oxide. The meltingproperties deteriorate if the proportion thereof is lower than 65 wt %.If the proportion thereof exceeds 80 wt %, the composition becomesstable as glass so that the crystal is less likely to be precipitated.

Al₂O₃ is a glass intermediate oxide and a component of an aluminumborate crystal, which is a crystalline phase precipitated by heattreatment. If the proportion of Al₂O₃ is lower than 3 wt %, the crystalis precipitated in reduced quantity and a sufficient strength is notachieved. If the composition rate of Al₂O₃ exceeds 15 wt %, the meltingtemperature is increased and devitrification is more likely to occur.

Li₂O is a fluxing agent and a component a component of a lithiumdisilicate crystal, which is a crystalline phase precipitated by heattreatment. If the proportion of Li₂O is lower than 3 wt %, theprecipitation amount of lithium disilicate crystal is insufficient. Ifthe composition rate of Li₂O exceeds 15 wt %, a lithium disilicatecrystal, which is a crystalline phase precipitated by heat treatment isnot stable so that the crystallization process cannot be controlled. Inaddition, the chemical durability is reduced, which may affect amagnetic film, while the stability in the polishing to cleaning steps islowered.

P₂O₅ is a fluxing agent and a nuclear forming agent for precipitating asilicate crystal, which is an important component for uniformlyprecipitating the crystal over the entire glass. If the proportion ofP₂O₅ is lower than 0.2 wt %, satisfactory nuclei are less likely to beformed so that crystal grains are increased in size or the crystal isprecipitated non-uniformly. Consequently, an extremely small and uniformcrystal structure is less likely to be obtained and a flat, smoothsurface required of the glass substrate as a disk substrate cannot beobtained by polishing. If the proportion of P₂O₅ exceeds 5 wt %, thereactivity of the glass in a molten state to a filter medium isincreased and the devitrifiability thereof is also increased, so thatproductivity during melt molding is reduced. In addition, the chemicaldurability is reduced, which may affect a magnetic film, while thestability in the polishing to cleaning steps is lowered.

ZrO₂ is a glass intermediate oxide and a nuclear forming agent. Inparticular, ZrO₂ holds the precipitation of a quartz crystal in crystal.If the proportion of ZrO₂ is lower than 0.5 wt %, satisfactory crystalnuclei are less likely to be formed so that crystal grains are increasedin size and the crystal is precipitated non-uniformly. This prevents theobtention of an extremely small and uniform crystal structure and theobtention of a flat, smooth surface by polishing, which is required ofthe glass substrate as a disk substrate. In addition, the chemicaldurability and the migration resistance are reduced. This may affect amagnetic film and degrades stability in the polishing to cleaning steps.If the proportion of ZrO₂ exceeds 0.8 wt %, the melting temperature isincreased and devitrification is more likely to occur during meltmolding, which lowers productivity. Moreover, the precipitatedcrystalline phase changes so that desired characteristics are lesslikely to be obtained.

Besides the above-mentioned basic components, CaO as a fluxing agent canbeen added. By adding CaO serving as a fluxing agent, a melting propertyand a stable crystal phase are improved. If the proportion of CaO islower than 0.1 wt %, a melting property does not sufficiently improve.If the proportion thereof exceeds 5 wt %, the composition becomes stableas glass so that the crystal is less likely to be precipitated and asufficient strength is not achieved.

Besides the above-mentioned basic components, K₂O as a fluxing agent canbeen added. By adding K₂O serving as a fluxing agent K₂O, the productionstability is improved. If the proportion of K₂O is lower than 0.1 wt %,however, the melting properties are not improved sufficiently. If theproportion of K₂O exceeds 5 wt %, the glass becomes stable and thecrystallization is suppressed, while the chemical durability is reduced.This may affect a magnetic film and degrades stability in the polishingto cleaning steps.

Besides the above-mentioned basic components, Sb₂O₃ as a fluxing agentcan been added. By adding Sb₂O₃ serving as a fluxing agent, productionstability has been improved. If the proportion of Sb₂O₃ is lower than0.1 wt %, however, a sufficient clarifying effect can not be achievedand the productivity is lowered. If the proportion of Sb₂O₃ exceeds 5 wt%, the crystallization of the glass becomes unstable and theprecipitated crystalline phase cannot be controlled so that requiredcharacteristics are less likely to be obtained.

Besides the above-mentioned basic components, B₂O₃ as a former can beenadded. By adding B₂O₃ serving as a former, the phase splitting of theglass is promoted and the precipitation and growth of the crystal arepromoted. If the proportion of B₂O₃ is lower than 0.1 wt %, the meltingproperties are not improved sufficiently. If the proportion of B₂O₃exceeds 15 wt %, devitrification is more likely to occur and moldingbecomes difficult, while the crystal is increased in size, so that anextremely small crystal is no more obtained.

Besides the above-mentioned basic components, MgO as a fluxing agent canbeen added. By adding MgO serving as a fluxing agent, the crystal in theform of grains aggregates to form an aggregation of crystal grains. Ifthe proportion of MgO is lower than 0.1 wt %, the range of operatingtemperatures is narrowed down and the chemical durability of a glassmatrix phase is not improved. If the proportion of MgO exceeds 12 w %,another crystalline phase is precipitated so that it becomes difficultto achieve a desired strength.

Besides the above-mentioned basic components, BaO as a fluxing agent canbeen added. By adding BaO serving as a fluxing agent, productionstability has been improved. If the proportion of BaO is lower than 0.1wt %, however, the melting properties are not improved sufficiently. Ifthe proportion of BaO exceeds 5 wt %, the glass becomes stable and thecrystallization is suppressed so that it becomes difficult to achieve adesired strength.

Besides the above-mentioned basic components, ZnO as a fluxing agent canbeen added. By adding ZnO serving as a fluxing agent, it helps uniformprecipitation of the crystal. If the proportion of ZnO is lower than 0.1wt %, however, the uniformity of the crystal is not sufficientlyimproved. If the proportion of ZnO exceeds 5 wt %, the glass becomesstable and the crystallization is suppressed, so that required strengthis less likely to be achieved.

Besides the above-mentioned basic components, Nb₂O₅ as a fluxing agentcan been added. By adding Nb₂O₅ serving as a fluxing agent, a materialserving as a crystal nucleating agent is increased. If the proportion ofNb₂O₅ is lower than 0.1 wt %, the rigidity is not sufficiently improved.If the proportion of Nb₂O₅ exceeds 5 wt %, the crystallization of theglass becomes unstable and the precipitated crystalline phase cannot becontrolled, so that desired characteristics are less likely to beobtained.

Besides the above-mentioned basic components, Ta₂O₅ as a fluxing agentcan been added. By adding Ta₂O₅ serving as a fluxing agent, the meltingproperties and strength are improved, while the chemical durability ofthe glass matrix phase is improved. If the proportion of Ta₂O₅ is lowerthan 0.1 wt %, however, the rigidity is not sufficiently improved. Ifthe proportion of Ta₂O₅ exceeds 5 wt %, the crystallization of the glassbecomes unstable and the precipitated crystalline phase cannot becontrolled, so that desired characteristics are less likely to beobtained.

Besides the above-mentioned basic components, La₂O₃ as a fluxing agentcan been added. By adding La₂O₃ serving as a fluxing agent, theprecipitation of the crystal is suppressed. If the proportion of La₂O₃is lower than 0.1 wt %, however, the rigidity is not improvedsufficiently. If the proportion of La₂O₃ exceeds 5 wt %, thecrystallization of the glass becomes unstable and the precipitatedcrystalline phase cannot be controlled, so that required properties areless likely to be obtained.

Next, a description will be given to a fabrication method. Raw materialscontaining the main components of the glass substrate to be finallyproduced are sufficiently mixed in specified proportions. The resultingmixture is placed in a platinum crucible and caused to melt. The moltenproduct is cast in a metal mold so that it is formed into a roughconfiguration and annealed to a room temperature. The molten product isthen held at a specified temperature for a specified time during aprimary treatment (heat treatment) such that crystal nuclei are formed.Subsequently, the molded mixture is held at a specified temperature fora specified time during a secondary heat treatment such that crystalnuclei grow. By slowly cooling the molded mixture, an objectivecrystallized glass is obtained.

NUMERICAL EXAMPLES

A description will be given next to specific numerical examplesincorporating the embodiments. In Table 1: the proportions(unit: wt %)of materials composing the glasses of the examples 1-4; the meltingtemperatures and times; the primary heat treatment temperatures andtimes; the secondary heat treatment temperatures and times; the mainprecipitated crystalline phases; the subordinate precipitatedcrystalline phases; the mean diameters of the crystal grains; thespecific gravity s: the Young's moduli; and the specific moduli.Likewise, the glasses of the examples 5-8 are shown in Table 2.Likewise, the glasses of the examples 9-12 are shown in Table 3.Likewise, the glasses of the examples 13-16 are shown in Table 4.Likewise, the glasses of the examples 17-20 are shown in Table 5.Likewise, the glasses of the examples 21-23 are shown in Table 6.

TABLE 1 Example 1 Example 2 Example 3 Example 4 SiO₂ 75.2 74.0 76.0 75.0Al₂O₃ 6.5 10.4 10.5 10.5 Li₂O 8.5 11.0 9.0 9.5 P₂O₅ 4.0 4.5 3.8 4.0 ZrO₂0.5 0.1 0.7 0.8 CaO 2.5 0.2 K₂O 1.8 Sb₂O₃ 1.0 Melting Temperatures 14601440 1440 1440 (° C.) Melting Times (hours) 2.50 2.50 2.50 2.50 PrimaryHeat Treatment 570 570 575 570 Temperatures (° C.) Primary HeatTreatment 5.50 5.50 5.50 5.50 Times (hours) Secondary Heat 690 700 690700 Treatment Temperatures (° C.) Secondary Heat 2.50 3.00 2.50 3.00Treatment Times (hours) Main Precipitated Lithium Lithium LithiumLithium Crystalline Phase disilicate disilicate disilicate disilicateSubordinate Precipitated Quartz Quartz Quartz Quartz Crystalline PhaseMean Diameters of the 0.06 0.06 0.06 0.06 Crystal Grains (μm) SpecificGravity 2.40 2.21 2.28 2.28 Young's Module 93.0 88.0 88.1 88.1 SpecificModule 38.7 39.8 38.7 38.7

TABLE 2 Example 5 Example 6 Example 7 Example 8 SiO₂ 74.0 73.0 69.0 75.0Al₂O₃ 10.5 11.0 12.0 10.0 Li₂O 9.0 10.6 12.0 9.5 P₂O₅ 3.3 4.2 2.3 3.9ZrO₂ 0.5 0.7 0.2 0.6 CaO 2.7 K₂O 0.5 4.5 Sb₂O₃ 1.0 Melting Temperatures1440 1440 1440 1460 (° C.) Melting Times (hours) 2.50 2.50 2.50 2.50Primary Heat Treatment 575 570 580 575 Temperatures (° C.) Primary HeatTreatment 5.50 5.50 5.00 5.50 Times (hours) Secondary Heat 700 700 720700 Treatment Temperatures (° C.) Secondary Heat 3.00 3.00 4.00 3.00Treatment Times (hours) Main Precipitated Lithium Lithium LithiumLithium Crystalline Phase disilicate disilicate disilicate disilicateSubordinate Precipitated Quartz Quartz Quartz Quartz Crystalline PhaseMean Diameters of the 0.08 0.08 0.08 0.08 Crystal Grains (μm) SpecificGravity 2.29 2.25 2.23 2.34 Young's Module 88.0 88.0 87.0 87.2 SpecificModule 38.4 39.0 38.9 37.2

TABLE 3 Example Example Example Example 9 10 11 12 SiO₂ 69.0 74.0 71.074.0 Al₂O₃ 10.5 9.5 11.4 11.0 Li₂O 11.0 10.0 9.0 9.6 P₂O₅ 5.0 3.8 4.24.2 ZrO₂ 0.5 0.7 0.4 0.7 Sb₂O₃ 4.0 B₂O₃ 2.0 4.0 MgO 0.5 MeltingTemperatures 1460 1440 1440 1440 (° C.) Melting Times (hours) 2.50 2.502.50 2.50 Primary Heat Treatment 570 575 570 570 Temperatures (° C.)Primary Heat Treatment 5.50 5.50 5.50 5.50 Times (hours) Secondary Heat720 700 700 700 Treatment Temperatures (° C.) Secondary Heat 4.00 3.003.00 3.00 Treatment Times (hours) Main Precipitated Lithium LithiumLithium Lithium Crystalline Phase disilicate disilicate disilicatedisilicate Subordinate Precipitated Quartz Quartz Quartz QuartzCrystalline Phase Mean Diameters of the 0.08 0.08 0.08 0.08 CrystalGrains (μm) Specific Gravity 2.56 2.23 2.20 2.27 Young's Module 88.087.0 86.2 86.8 Specific Module 34.4 39.0 39.1 38.3

TABLE 4 Example Example Example Example 13 14 15 16 SiO₂ 72.0 76.0 72.075.0 Al₂O₃ 10.0 9.5 10.0 9.5 Li₂O 9.5 9.0 9.5 9.2 P₂O₅ 3.7 3.7 4.3 3.6ZrO₂ 0.3 0.8 0.2 0.7 MgO 4.5 BaO 1.0 4.0 ZnO 2.0 Melting Temperatures1440 1460 1500 1460 (° C.) Melting Times (hours) 2.50 2.50 2.50 2.50Primary Heat Treatment 575 575 570 575 Temperatures (° C.) Primary HeatTreatment 5.50 5.50 5.50 5.50 Times (hours) Secondary Heat 700 690 700700 Treatment Temperatures (° C.) Secondary Heat 3.00 2.50 3.00 3.00Treatment Times (hours) Main Precipitated Lithium Lithium LithiumLithium Crystalline Phase disilicate disilicate disilicate disilicateSubordinate Precipitated Quartz Quartz Quartz Quartz Crystalline PhaseMean Diameters of the 0.08 0.08 0.08 0.08 Crystal Grains (μm) SpecificGravity 2.23 2.38 2.62 2.34 Young's Module 87.0 87.2 91.0 88.0 SpecificModule 39.0 36.7 34.8 37.6

TABLE 5 Example Example Example Example 17 18 19 20 SiO₂ 76.0 77.0 76.075.0 Al₂O₃ 7.6 9.7 8.6 13.3 Li₂O 9.8 9.6 9.5 9.0 P₂O₅ 3.3 2.5 2.0 1.5ZrO₂ 0.3 0.7 0.4 0.7 ZnO 3.0 Nb₂O₅ 0.5 3.5 Ta₂O₅ 0.5 MeltingTemperatures 1460 1440 1460 1460 (° C.) Melting Times (hours) 2.50 2.502.50 2.50 Primary Heat Treatment 575 580 580 585 Temperatures (° C.)Primary Heat Treatment 5.50 5.00 5.00 5.00 Times (hours) Secondary Heat690 690 690 700 Treatment Temperatures (° C.) Secondary Heat 2.50 2.502.50 3.00 Treatment Times (hours) Main Precipitated Lithium LithiumLithium Lithium Crystalline Phase disilicate disilicate disilicatedisilicate Subordinate Precipitated Quartz Quartz Quartz QuartzCrystalline Phase Mean Diameters of the 0.08 0.08 0.08 0.08 CrystalGrains (μm) Specific Gravity 2.34 2.29 2.44 2.34 Young's Module 88.287.8 90.0 88.8 Specific Module 37.7 38.3 36.8 38.0

TABLE 6 Example Example Example 21 22 23 SiO₂ 74.0 74.0 77.0 Al₂O₃ 12.511.0 10.5 Li₂O 8.0 10.6 7.3 P₂O₅ 1.5 3.5 2.5 ZrO₂ 0.2 0.7 0.2 Ta₂O₅ 3.8La₂O₃ 0.2 2.5 Melting Temperatures (° C.) 1520 1440 1460 Melting Times(hours) 1.75 2.50 2.50 Primary Heat Treatment 585 575 580 Temperatures(° C.) Primary heat Treatment 5.00 5.50 5.00 Times (hours) SecondaryHeat Treatment 700 700 690 Temperatures (° C.) Secondary Heat Treatment3.00 3.00 2.50 Times (hours) Main Precipitated Crystalline Phase LithiumLithium Quartz Subordinate Precipitated Crystalline disilicatedisilicate Lithium Phase Quartz Quartz disilicate Mean Diameters of theCrystal 0.08 0.08 0.08 Grains (μm) Specific Gravity 2.76 2.27 2.52Young's Module 91.0 86.2 89.0 Specific Module 33.0 38.0 35.4

Although the present invention has been fully described by way ofexample with reference to the accompanying drawings, it is to beunderstood that various changes and modifications will be apparent tothose skilled in the art. Therefore, unless otherwise such changes andmodification depart from the scope of the present invention, they shouldbe construed as being included therein.

What is claimed is:
 1. A glass ceramics composition for recording disksubstrate consisting essentially, expressed in terms of weight percenton the oxide basis, of from 65 to 80 wt % of SiO₂, from 6.5 to 15 wt %of Al₂O₃, from 3 to 15 wt % of Li₂O, from 0.2 to 5 wt % of P₂O₅ and from0.1 to 0.8 wt % of ZrO₂.
 2. A glass ceramics composition as claimed inclaim 1, wherein said glass ceramic composition further consistsessentially, expressed in terms of weight percent on the oxide basis, offrom 0.1 to 5 wt % of CaO.
 3. A glass ceramics composition as claimed inclaim 1, wherein said glass ceramic composition further consistsessentially, expressed in terms of weight percent on the oxide basis, offrom 0.1 to 5 wt % of K₂O.
 4. A glass ceramics composition as claimed inclaim 1, wherein said glass ceramic composition further consistsessentially, expressed in terms of weight percent on the oxide basis, offrom 0.1 to 5 wt % of Sb₂O₃.
 5. A glass ceramics composition as claimedin claim 1, wherein said glass ceramic composition further consistsessentially, expressed in terms of weight percent on the oxide basis, offrom 0.1 to 15 wt % of B₂O₃.
 6. A glass ceramics composition as claimedin claim 1, wherein said glass ceramic composition further consistsessentially, expressed in terms of weight percent on the oxide basis, offrom 0.1 to 12 wt % of MgO.
 7. A glass ceramics composition as claimedin claim 1, wherein said glass ceramic composition further consistsessentially, expressed in terms of weight percent on the oxide basis, offrom 0.1 to 5 wt % of BaO.
 8. A glass ceramics composition as claimed inclaim 1, wherein said glass ceramic composition further consistsessentially, expressed in terms of weight percent on the oxide basis, offrom 0.1 to 5 wt % of ZnO.
 9. A glass ceramics composition as claimed inclaim 1, wherein said glass ceramic composition further consistsessentially, expressed in terms of weight percent on the oxide basis, offrom 0.1 to 5 wt % of Nb₂O₅.
 10. A glass ceramics composition as claimedin claim 1, wherein said glass ceramic composition further consistsessentially, expressed in terms of weight percent on the oxide basis, offrom 0.1 to 5 wt % of Ta₂O₅.
 11. A glass ceramics composition as claimedin claim 1, wherein said glass ceramic composition further consistsessentially, expressed in terms of weight percent on the oxide basis, offrom 0.1 to 5 wt % of La₂O₃.
 12. A glass ceramic recording disksubstrate, wherein the glass ceramic is prepared from a compositionconsisting essentially, expressed in terms of weight percent on theoxide basis, of from 65 to 80 wt % of SiO₂, from 6.5 to 15 wt % ofAl₂O₃, from 3 to 15 wt % of Li₂O, from 0.2 to 5 wt % of P₂O, and from0.1 to 0.8 wt % of ZrO₂.
 13. A recording disk substrate as claimed inclaim 12, wherein said composition further consists essentially,expressed in terms of weight percent on the oxide basis, of from 0.1 to5 wt % of CaO.
 14. A recording disk substrate as claimed in claim 12,wherein said composition further consists essentially, expressed interms of weight percent on the oxide basis, of from 0.1 to 5 wt % ofK₂O.
 15. A recording disk substrate as claimed in claim 12, wherein saidcomposition further consists essentially, expressed in terms of weightpercent on the oxide basis, of from 0.1 to 5 wt % of Sb₂O₃.
 16. Arecording disk substrate as claimed in claim 12, wherein saidcomposition further consists essentially, expressed in terms of weightpercent on the oxide basis, of from 0.1 to 15 wt % of B₂O₃.
 17. Arecording disk substrate as claimed in claim 12, wherein saidcomposition further consists essentially, expressed in terms of weightpercent on the oxide basis, of from 0.1 to 12 wt % of MgO.
 18. Arecording disk substrate as claimed in claim 12, wherein saidcomposition further consists essentially, expressed in terms of weightpercent on the oxide basis, of from 0.1 to 5 wt % of BaO.
 19. Arecording disk substrate as claimed in claim 12, wherein saidcomposition further consists essentially, expressed in terms of weightpercent on the oxide basis, of from 0.1 to 5 wt % of ZnO.
 20. Arecording disk substrate as claimed in claim 12, wherein saidcomposition further consists essentially, expressed in terms of weightpercent on the oxide basis, of from 0.1 to 5 wt % of Nb₂O₅.
 21. Arecording disk substrate as claimed in claim 12, wherein saidcomposition further consists essentially, expressed in terms of weightpercent on the oxide basis, of from 0.1 to 5 wt % of Ta₂O₅.
 22. Arecording disk substrate as claimed in claim 12, wherein saidcomposition further consists essentially, expressed in terms of weightpercent on the oxide basis, of from 0.1 to 5 wt % of La₂O₃.
 23. Arecording disk substrate as claimed in claim 12, wherein the glassceramic includes crystalline phase of quartz.
 24. A recording disksubstrate as claimed in claim 12, wherein the glass ceramic includescrystalline phase of lithium disilicate.
 25. A recording disk substrateas claimed in claim 12, wherein the recording disk substrate contains alithium disilicate as a main crystalline phase or subordinatecrystalline phase after heat treatment.